78 Papers from the Marine Biological Laboratory at Tortugas. 



tively thicker and more easy to trace, and the specimen shows clearly that 

 muscles 13 and 14 are not united where they cross each other. The younger 

 embryo that is shown, in side view, in plate 2, figure 7, shows the muscle 16 

 that joins muscle i to muscle 5 on the side of the body. This muscle is 

 difficult to see in a dorsal or ventral view. The left-hand one of the adult 

 is shown, at 16, on the left side of figure i and the right-hand one is shown 

 on the left side (right side inverted) of figure 2. 



Since the muscles of 5". floridana are homologous with, but much more 

 specialized than, those of S. pinnata, it seems natural to conclude that it is 

 a modified descendant of an ancestral form that resembled S. pinnata, and 

 this conclusion is strengthened by the comparative study of other organs. 



Apstein says that there is no other Salpa in which the muscles are 

 joined to each other in as great a degree as they are in vS". floridana, but a 

 reference to Traustedt's figure of S. hexagona, will show that all the muscles 

 are thus united in this species. (Traustedt, Spolia Atlantica, Tab. i, fig. 14.) 



THE DIGESTIVE ORGANS AND STOLON OF THE SOLITARY SALPA FLORIDANA. 



(Plate I, fig. 3.) 



The opening- by which the pharynx communicates with the oesophagus 

 is a large, funnel-shaped aperture, on the right side of the ventral end of 

 the " gill." It diminishes in size very rapidly, and opens into the anterior 

 end of the stomach by an opening, c, which is very small as compared with 

 the pharyngeal end. The stomach is joined, on the left, by a large blind 

 pouch, the so-called liver. The aperture by which the stomach communi- 

 cates with the intestine, i, is at the junction of the blind pouch with the 

 stomach. The intestine, i, is long, and it runs upwards and forwards to 

 open at the anus, a, into the median atrium or cloaca, near its dorsal surface 

 and a little posterior to the ganglion. The intestine lies in that part of the 

 body-cavity that is included in the so-called " gill." 



The stolon is twisted into a right-hand spiral, bending to the left from 

 the fixed growing end of the median line and then bending to the right, 

 so that the free end points to the right and is on the right of the middle 

 line. In this respect the stolon of this species is very different from that 

 of 5\ pinnata, and like that of ordinary salpas, such as 5. democratica. 



THE AGGREGATED FORM OF SALPA FLORIDANA. 

 (Plate i, figs. 5 and 6; plate 2, fig. 9.) 



The colony of the aggregated form of S. floridana (plate 2, figure 9) is 

 a circular rosette of four, five, or six or more individuals. In my collection 

 there are some with four, some with five, and one with six, and none with 

 more than six, although six may not be the maximum. The number is 

 very much smaller than it is in other Cyclosalpas, as is shown by comparing 

 figure 9 with the colony of S. pinnata shown in plate 2, figure 8. The 



